Silver halide emulsions are generally made in solutions containing a low concentration of a colloid stabilizer. Generally, gelatin is found to be the most suitable stabilizer since it also serves as a film former in photographic coatings. The silver halide is a product of a double decomposition reaction of the respective halide of an alkali metal and silver nitrate. The resulting precipitation reaction results in a colloidal suspension of silver halide, which contains a considerable amount of water and high levels of salt (nitrates). In order to minimize drying loads during the coating operation, it is advantageous to remove water from the emulsion. Secondly, the high salt contents can impart deleterious properties to coating solutions and the coated products. Thus, some amount of deionization of the emulsion is required. Desalination and deionization of the emulsion can be achieved by several methods. Methods for deionization include dialysis, electrodialysis, ion exchange and washing of noodles. Methods for concentration include ultrafiltration and evaporation. A third method, coagulation, can achieve deionization and concentration simultaneously. Gelatin coagulants include derivatized gelatin, anionic polymers, inorganic salts, esters of phosphoric acid and dicarboxylic acids. Generally, these coagulants flocculate gelatin when the pH of the emulsion is decreased close to or below the isoelectric point of gelatin, which is 4.9 for lime processed ossein gelatin.
Anionic polymers as emulsion coagulants are the generally preferred type of coagulant. The coagulation procedure involves addition of polymer at levels from 2-50% with respect to the gelatin present. The pH is then lowered slightly below the coagulation point of the system. The separation of phases is effected by gravity or centrifugation. The supernatant containing water, salts and some gelatin is separated from the coagulate which contains the emulsion, gelatin and remaining salts. At this point, generally, not enough of the salt amount is removed from the emulsion. In order to remove the desired amounts of salts, the flocculated emulsion is washed with water in one or more subsequent steps. The deionized emulsion is then resuspended in water and gelatin by raising the pH above the isoelectric point of gelatin. As with all technologies, there are a few disadvantages with this method--1) the polymer coagulant remains with the emulsion and ends up in the coated product. This could result in potential problems, such as adversely affecting the sensitometric characteristics of the emulsion, viscosifying the coating melts etc. 2) Lowering and raising the pH to effect coagulation and resuspension, respectively, can have adverse effects. For example, the addition of base to an emulsion, frequently causes changes in the sensitometric characteristics, due to R-typing. Thus, the amount of base needed is inversely proportional to the pH of coagulation. A reduction in the amount of polymer required for coagulation and/or an increase in the pH of coagulation would be a desirable improvement on this technology. Another improvement would be to eliminate the need for multiple washing. Multiple washing requires a large amount of process time. Secondly, an emulsion that has been multiply washed and separated is found to be hard to resuspend. Specifically, small pieces of coagulum remain even after substantial mixing at the resuspension step.
The use of anionic polymers for concentrating emulsions is well known in the art. U.S. Pat. No. 2,772,165 discloses acid soluble polymers to coagulate emulsions. GB 884,840 discloses polystyrene sulfonate as a coagulating agent. Carboxylic acid containing polymers are disclosed in U.S. Pat. No. 2,565,418. Maleic acid copolymers with hydrophobic nonionic comonomers have been disclosed in U.S. Pat. No. 4,087,282, GB 1,121,188 and U.S. Pat. No. 5,411,849. Copolymers containing sulfonic acid and carboxylic acid groups have been disclosed in U.S. Pat. No. 5,486,451. All these polymers suffer from the above-mentioned problems, albeit to varying degrees. Anionic polymers containing amine groups, either on the same monomer as the anionic moiety or as a comonomer, are disclosed in U.S. Pat. No. 3,178,294; U.S. Pat. No. 3,455,694; U.S. Pat. No. 3,482,980; GB 889,760; and GB 911,886. These polymers are alleged to have a high pH of coagulation with gelatin. However, due to the interaction of both the anionic moiety and the amine group with gelatin, the dissolution of the coagulum is not easily achieved via pH control. Although, these polymers are able to raise the pH of coagulation, they still require relatively high amounts of polymer to achieve phase separation. U.S. Pat. No. 3,884,701 discloses the combination of an anionic polymer with phthalated gelatin as a coagulant for an emulsion prepared with regular gelatin, to reduce the amount of polymer that is needed. However, the process of derivatizing gelatin is relatively expensive and also the use of derivatized gelatin results in a large amount of variability to the coagulation process. None of the prior art disclosed above addresses the problem of having to carry out multiple washing steps.